Visual field testing also referred to as "visual field analysis" is a fundamental and important part of both routine and specialized eye examinations. According to historians in this art, visual field defects have been noted for centuries beginning perhaps as early as Hippocrates (circa 460-380 B.C.), and the techniques, observations and interpretations of visual field analysis have continued throughout medical history. For at least the last century, it has been well understood that the visual field is as important a measurement of a patient's visual function as is central visual acuity. As understood by those of ordinary skill in this art, the normal visual field of an eye is a slightly irregular oval. From its center, it extends approximately 50.degree. in the superior direction (towards the eyebrow), about 70.degree. in the inferior direction (towards the feet), about 90.degree.-100.degree. temporally (toward the ear), and about 60.degree. nasally (toward the nose). When the visual field of a patient differs substantially from these norms, particularly in a decreasing fashion, some associated medical problem is most likely indicated. Problems that are exhibited by a change in the visual field include glaucoma, neurologic problems such as lesions of the chiasm, optic tract defects, temporal lobe lesions, parietal lobe lesions, occipital lobe lesions, toxic amblyopias, and other non-neurological diseases including various diseases of the retina, choroid and media. Visual field analysis is also an important test in the diagnosis and management of other diseases including diabetes, brain tumors, and cerebro-vascular accidents.
The visual field of a patient can be tested in various manners, but as suggested by the nature of the visual field, these techniques generally comprise methods of measuring the points at which a patient's eye can discriminate certain sensory input such as flashes of light within the visual field. Basically, a patient's visual field is analyzed by moving a test spot such as an object or point of light from an area of non-vision to an area of vision (kinetic techniques) or by increasing the test spot intensity until it can be seen at a fixed location (static techniques).
Typical testing methods include "confrontations" in which a patient is asked to detect the examiner's fingers in different quadrants of vision at a distance of about 1 meter; tangent screen, in which the patient is usually seated at a distance of about 1 meter from a black screen while a target of varying dimensions is manually moved through the different quadrants of vision; and bow perimetry. In bowl perimetry, the patient is seated with his head positioned on a chin rest with the head extending into a lighted hemisphere. One eye is covered and the patient is required to fixate on a stationary or moving target and to signal when he detects a light flash (stationary testing) or movement into or out of (moving target) the visual field.
Because of the nature of human vision in which the eyes work together and are coordinated by the brain to produce the images perceived by the subject, and because the visual fields of each eye overlap, the task of measuring the visual field of one eye requires that the other eye be covered or occluded during the testing procedure. The untested eye is thus generally covered with an opaque device such as an eye patch in order to prevent as much light as possible from entering the covered eye and interfering with the test.
Recently, S. J. Bolanowski and R. J. Doty have reported in perceptual "Blankout" of Monocular Homogenous Fields (Ganzfelder) Is Prevented With Binocular Viewing, Vision Res. Vol. 27, No. 6, pp. 967-982, 1987, that when one eye is presented with darkness and the other is presented with a homogenous light field (a "Ganzfeld"), a loss of visual perception in the form of an intermittent darkening or "blankout" occurs in the eye presented with the light field. The blankout phenomenon has been reported as "a dramatic vanishing of visual sensation despite continuing simulation." This blankout is so persistent that it has been used to test the sufficiency of adequate Ganzfeld illumination and retinal stabilization. Bolanowski and Doty were able to determine that such Ganzfeld blankout could be minimized or eliminated if a sufficient amount of illumination was provided to each eye. They did not, however, perform any visual field testing, did not occlude either eye, and did not test visual acuity. Moreover, a Ganzfeld provides no clues to depth or depth accommodation, and Bolanowski's and Doty's experiment gave no indication that the blankout phenomena would occur during visual testing in general or visual field testing in particular.
Therefore, prior to the present invention, the phenomena of blankout or its cure had not been addressed with respect to the important technique of visual field testing. As described further herein with respect to the present invention, it has now been discovered that blankout occurs during visual field testing. Because of the diagnostic importance of visual field testing, there exists the need for procedures and devices that can minimize or eliminate the blankout phenomena during such testing.